GRADING AND SCREENING 259
For ordinary revolving screens punched plate will be found better than cloth down to about l i-in. openings. The percentage of opening of the punched plate is less than with the* cloth but tin; plate is more easily attached to and held on the screen supports. Silk cloth is preferred to metal even in some industries where metal cloth would at first thought seem to have the preference. One very prominent manufacturer of abrasives of grout hardness prefers silk to brass cloth because it does not blind so readily. Moisture and sudden temperatures are destructive to silk cloth, the first because cloth and gauws are held together with size. A sudden lowering of temperature will often cause a tightly Ktretched silk cloth to tear. An accompanying table gives the meshes of cloth nncl gauzes and also the equivalency in opening between cloth and X grit/, gauze, ami X and XXX gritx gauze.
Screen Ratio for Testing Screens.—The advantage of the screen ratio 1.414 is stated in the words of the* following quotation concerning it from a W. S. Tyler trade publication: "The ratio between the different sizes of the scale has been taken as 1.414 or the square root of 2, as recommerj Kitlingcr in his work on ore dressing. The nicenoas of this will be ap^ frnm tin* following: inking 0.0029 in. or 0.074 mm., the opening in the 200 sieve as tin1 !>asr or Html ing point, the diameter of each successive opem exactly 1.414 times the opening in the previous sieve. It also makes the or surface of each successive opening in the scale just double that of the : liner or half that of the six.es have* a constant ratio of 1.414 while the areas of the successive opening" have a constant ratio of 2.
"Another advantage in this selection of ratio is that by skipping every other wreen y«»u have a ratio of diameter of 2:1, by skipping two sixes you have a ratio of II: I (approximately», and by skipping three sizes, you get a ratio of 4:1." The equivalent for the ratio in the* ordinary range of screen cloths for testing is shown by flit* accompanying table. These arc all standard Tyler screen sizes.
Where a closer ni/ing in required in the finer openings, a scale is shown below this tfthle from US to 2IMI mesh, in which the openings inc-reane in the ratio of the fourth root of 2 or l.IKil,